Automatic correction of loudness level in audio signals

ABSTRACT

Systems and method for adapting a gain of an audio signal containing at least two different tracks with different signal level ranges. An example system includes a loudness determination unit for determining a perceived loudness of the audio input signal based on a psychoacoustic model of a human hearing. The system further includes a loudness adaptation unit configured to determine a loudness adaptation indicator based on the perceived loudness and a predetermined loudness, and to introduce the loudness adaptation indicator into the audio signal to generate an adapted gain signal. A gain determination unit is configured to adapt the gain for outputting the audio signal containing the at least two tracks based on the loudness adaptation indicators in the adapted gain signal at about the predetermined loudness.

RELATED APPLICATIONS

This application claims priority of European Patent Application SerialNo. 10 156 409.4 filed Mar. 12, 2010 titled AUTOMATIC CORRECTION OFLOUDNESS LEVEL IN AUDIO SIGNALS, and European Patent Application SerialNo. 10 197 304.8 filed Dec. 29, 2010 titled AUTOMATIC CORRECTION OFLOUDNESS LEVEL IN AUDIO SIGNALS, which applications are incorporated intheir entirety by reference in this application.

BACKGROUND

1. Field of the Invention

This invention relates to audio signal processing and, in particular, tomethods and systems for adapting a gain of an audio output signal.

2. Related Art

Audio systems may process audio from many different sources of audiosignals that may contain music and/or speech. Audio signals containingmusic may be stored on a CD, a DVD or on any other suitable storagemedium. Recent developments in compression schemes such as MPEG allowfor audio signals with music and/or speech from different genres andartists to be stored on a storage medium and combined in a playlist tobe played out to a user. The audio signals are typically derived fromdifferent audio sources having different signal and dynamics compressionlevels. The audio sources may be from different tracks that havedifferent signal level ranges. When played out, for example in aplaylist, the different tracks may be perceived by the user as being atdifferent loudness levels.

In a vehicle environment, audio perceived by passengers may contain theaudio signal itself and noise, which may include road tire noise,aerodynamics noise and engine noise. Audio signals played in a vehicleenvironment should be perceivable to the user, which means that theaudio signal loudness should exceed the noise present in the vehicle.The overall audio level, which includes the audio signal and the noise,should not exceed a level that may result in hearing damage or bepainful to the listener.

Audio signals from different sources may be difficult to hear if thesignals are played at different loudness levels. The problem isaggravated in environments having loud ambient noise. There is a needfor systems that control audio signal output levels to permit a listenerto perceive the audio signals from different tracks at a consistentsound level. There is a need for systems that provide consistent soundlevels in environments having loud ambient noise.

SUMMARY

In view of the above, systems and methods are provided for adapting again of an audio signal containing at least two different tracks withdifferent signal level ranges. In an example method, a perceivedloudness of the audio signal based on a psychoacoustic model of a humanhearing is dynamically determined Loudness adaptation indicators aregenerated based on the perceived loudness and a predetermined loudnessrange. The loudness adaptation indicators are introduced into the audiosignal to generate an adapted audio signal. An estimated ambient noiselevel is determined. The audio signal gain is dynamically adapted foroutputting the audio signal containing the at least two tracks based onthe loudness adaptation indicators in the adapted audio signal with anaverage loudness within the predetermined loudness range. The adaptedgain is applied to the output audio signal according to the ambientnoise level.

An example system for adapting a gain of an audio signal containing atleast two different tracks with different signal level ranges includes aloudness determination unit for determining a perceived loudness of theaudio input signal based on a psychoacoustic model of a human hearing.The system further includes a loudness adaptation unit configured todetermine a loudness adaptation indicator based on the perceivedloudness and a predetermined loudness range, and to introduce theloudness adaptation indicator into the audio signal to generate anadapted gain signal. A gain determination unit is configured to adaptthe gain for outputting the audio signal containing the at least twotracks based on the loudness adaptation indicators in the adapted gainsignal within the predetermined loudness range. A noise estimator isused to determine an estimated ambient noise. A gain control unitdetermines the extent to which the adapted gain is applied to the audiooutput signal based on the estimated ambient noise level.

Other devices, apparatus, systems, methods, features and advantages ofthe invention will be or will become apparent to one with skill in theart upon examination of the following figures and detailed description.It is intended that all such additional systems, methods, features andadvantages be included within this description, be within the scope ofthe invention, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE FIGURES

The description below may be better understood by referring to thefollowing figures. The components in the figures are not necessarily toscale, emphasis instead being placed upon illustrating the principles ofthe invention. In the figures, like reference numerals designatecorresponding parts throughout the different views.

FIG. 1 is a schematic diagram illustrating the general components ofsound in a vehicle and a graph illustrating the effect of vehicle speedon vehicle in-cabin noise.

FIG. 2 shows graphs illustrating an example of an audio signal and theestimated loudness of the audio signal without gain adaptation.

FIG. 3 shows graphs illustrating operation with a dynamic gainadjustment of the audio signal.

FIG. 4 is a block diagram of an example of a system for adapting thegain of an audio signal.

FIG. 5 is a block diagram of an example of an audio analyzing unit thatmay be used in the system in FIG. 4.

FIG. 6 illustrates the introduction of time constants into the audiosignal.

FIG. 7 shows graphs illustrating example audio signals before and afterautomatic loudness adaptation.

FIG. 8 shows graphs illustrating another example of audio signals beforeand after automatic loudness adaptation.

FIG. 9 is a flowchart illustrating operation of an example method foradapting a gain of an audio signal.

DETAILED DESCRIPTION

Occupants of a vehicle may perceive different ambient noises in additionto audio signals that the occupants may wish to perceive. The noise mayvary with a variety of factors including the type of vehicle beingdriven. FIG. 1 is a schematic diagram illustrating the generalcomponents of sound 100 in a vehicle and a graph 120 illustrating theeffect of vehicle speed on vehicle in-cabin noise. The vehicle soundsignal 100 includes noise components 102 and an audio signal component104. Sound 100 is typically measured in decibels sound pressure level(“dB SPL”). The audio signal component 104 represents sound that thevehicle occupants have an interest in hearing, such as audio from anentertainment system (radio, CD player, MP3 player, and any othersuitable entertainment audio source). The noise signal component 102includes the ambient noise that may interfere with the user's ability tohear the audio signal components 104, such as, for example, road tirenoise, aerodynamic noise, or engine noise. Entertainment systems employamplifying components to amplify the audio output to the occupants. Theaudio output is amplified according to a “gain,” which is typically amultiplier used on the audio output signal levels to increase ordecrease the volume. The user is typically given control of the volumevia a knob, or other suitable user interface components.

In a vehicle, ambient noise may vary with a variety of factors. As shownin the graph 120 in FIG. 1, the vehicle speed is one such factor, whichmay also vary with different types of vehicles. The graph 120 in FIG. 1includes a first curve 122 showing the noise generated in a roadster orsports car, and a second curve 124 showing the noise generated by asport utility vehicle (“SUV”), both curves 122, 124 plotted against thevehicle speed. As shown in the graph 120, the noise can be as high asbetween 60 and 85 dB SPL at a typical range of vehicle speeds. Thehearing pain threshold is about 120 dB SPL. In order to ensurecomfortable hearing of audio signals, the audio output should be in therange of within about 20-40 dB SPL.

FIG. 2 shows graphs illustrating an example of an audio signal 200 andthe estimated loudness of the audio signal 220 without gain adaptation.The audio signal 200 is shown in full scale, meaning that 0 dB fullscale (0 dBFS) is assigned to the maximum possible signal level in thedigital domain. A first portion of the audio signal 202 varies betweenhigh and low level signals in the full range of the signal level. Asecond portion 204 varies between high and low levels in a much smallerrange of the signal level. A third portion 206 appears to saturate thesignal level. The first and second portions 202, 204 may represent audiosound from a music signal source and the third portion 206 may representnoise in the audio signal, such as white noise in an audio gap betweentracks that has been amplified at the audio output.

The estimated loudness of the audio signal 220 illustrates how occupantsmay perceive the audio signal 200. The first portion of the audio signal202 may be perceived as shown in a first loudness portion 222. Thesecond portion of the audio signal 204 may be perceived as shown in asecond loudness portion 224 that is less than the first loudness portion222. The third portion of the audio signal 206 may be perceived as shownin a third loudness portion 226 that is the highest of the threeportions. The loudness estimation may be performed using a variety ofmodels, one of which is described in Recommendation ITU-R BS. 1770-1(“Algorithms to Measure Audio Program Loudness and to a Peak AudioLevel”) incorporated by reference in its entirety. In the presentapplication loudness may be estimated through a binaural localizationmodel.

When played to vehicle occupants, some parts of the audio signal 202,such as the third portion 206, may be perceived as unpleasantly loud.Other parts of the audio signal 202, such as the second portion 204, maybe too low to be correctly perceived by the user. FIG. 3 shows graphsillustrating operation with a dynamic gain adjustment of the audiosignal 200. An estimated loudness 300 in FIG. 3 shows the first portionof the audio signal 202 perceived as shown in a first loudness portion302. The second portion of the audio signal 204 is perceived as shown ina second loudness portion 304. The third portion of the audio signal 206is perceived as shown in a third loudness portion 306. The portions ofthe estimated loudness 302, 304, 306 in FIG. 3 corresponding to theportions of the audio signal 202, 204, 206 are more comparableillustrating a more even perception of the different levels of audio.The estimated loudness 300 results from dynamic gain adaptation of theaudio signal 200. The dynamic gain adaptation may output the signalsamples in the first audio signal portion 202 at a first signal level,the signal samples in the second audio signal portion 204 at a secondsignal level higher than the first signal level, and the signal samplesof the third audio signal portion 206 at an attenuated level. Acomparison of the estimated loudness 220 in FIG. 2 with the estimatedloudness 300 in FIG. 3 illustrates that a listener's loudness evaluationrepresented by estimated loudness 300 in FIG. 3 would be preferred overthe loudness evaluation illustrated by the estimated loudness 220 inFIG. 2. The more even estimated loudness 300 in FIG. 3 provides asmoothed, relatively constant loudness compared to that of the loudness220 in FIG. 2.

In example implementations, the samples of audio signal 200 in FIG. 2may originate from different tracks of audio signals provided on asingle signal source. The signal source for multiple tracks, forexample, may be a storage medium (e.g. a hard disk). Different tracks orpieces of music may be from distinct sources. For example, a first trackmay be from a CD/DVD, whereas a second track may be played out from ahard disk on which music signals may be stored in a compressed formatsuch as MP3. Different tracks may also be stored on a hard disk in acompressed or non-compressed format.

FIG. 4 is a block diagram of an example of a system 400 for adapting thegain of an audio signal 402. The system 400 includes an audio signalanalyzing unit 404, a signal controller 406, and an audio transducer410. The audio signal analyzing unit 404 determines the loudness of theaudio signal 402, which may include an entertainment audio signal. Theentertainment or audio signal 402 may include a 2.0, 5.1 or 7.1 audiosignals or any other signals in a suitable audio format.

The loudness may be determined using a psycho-acoustical localizationmodel of the human hearing and by using signal statistical analysis. Theaudio signal 402 is input to the signal controller 406, which includes adelay element 430 and a gain control unit 432. The gain control unit 432determines an output gain to be applied to the audio signal based on again adaptation provided by the audio signal analyzing unit 404 and anun-processed, or raw, gain setting. The gain control unit 432 maydetermine, for example, the extent to which an adapted gain is used overthe raw gain setting on the audio output signal 414 that is output tothe audio transducer 410 or to a post-processing stage (not shown) basedon the ambient noise level. The system 400 may include a noise estimator420 to determine an ambient noise level that may, for example, be basedon the vehicle velocity.

The signal analyzing unit 404 in FIG. 4 includes a psycho-acousticalmodel loudness function 450 and a statistical signal model loudnessfunction 452. The psycho-acoustical model loudness function 450determines the estimated loudness, localization of sound, and whethernoise is present in the audio input signal as a dominant factor (forexample, during a pause in a track or a pause between two tracks). Thestatistical signal model loudness function 452 may be used as a secondbasis for determining or estimating the loudness and for determiningwhether a pause with noise is present in the audio signal 402. Forexample, the statistical model loudness function 452 may determine thesignal strength of the entertainment audio signal 402. Based on thepsycho-acoustical model function 450 alone, or in combination with thestatistical signal model function 452, a perceived loudness may bedetermined. The perceived loudness may then be used to dynamicallydetermine loudness adaptation indicators as described further below withreference to FIGS. 5 and 6.

The signal analyzing unit 404 includes a dynamic model function 454 thatuses the perceived loudness to determine loudness adaptation informationfor introduction into the audio signal 402. The dynamic model function454 also includes a gain determination function to determine an adaptinggain for the adapted audio signal formed by the audio signal 402 andintroduced loudness adaptation information. The signal analyzing unit404 outputs a gain adapted signal 412 to the gain control unit 432.

FIG. 5 is a block diagram of an example of an audio analyzing unit 500that may be used in the system 400 in FIG. 4. The audio signal analyzingunit 500 in FIG. 5 includes a loudness determination unit 502, aloudness adaptation unit 504, a pause detection unit 506, a trackdetection unit 508, and a gain determination unit 510. The loudnessdetermination unit 502 estimates a perceived loudness of the receivedaudio signal 402. The loudness determination unit 502 may determine theloudness using methods known in the art. In one example, the loudnessdetermination unit 502 uses the binaural model of human hearing fordetermining loudness and for determining whether and where the audiosignal 402 could be localized by a user when hearing the audio signal402. The binaural model simulates the spatial perception of the audiosignal and enables a determination of whether the audio signal containsmainly noise or any other input signal, such as music or speech. Thelocalization of the audio signal 402 may be performed as described inmore detail in the following references, which are incorporated hereinby reference:

-   -   (1) EP 1 522 868 A1;    -   (2) “Acoustical Evaluation of Virtual Rooms by Means of Binaural        Activity Patterns” by Wolfgang Hess et al. in Audio Engineering        Society Convention Paper 5864, 115th Convention, October 2003;    -   (3) W. Lindemann “Extension of a Binaural Cross-Correlation        Model by Contralateral Inhibition. I. Simulation of        Lateralization for Stationary Signals”, in Journal of Acoustic        Society of America, December 1986, p. 1608-1622, Vol. 80 (6).

The localization of the audio signals is used to distinguish noise fromother sound signals and for avoiding the output of the noise at anincreased gain if only noise is detected in the audio signal.

The loudness determination unit 502 may also use statistical signalprocessing to estimate the loudness of the audio signal 402 or to detectsignal pauses. A statistical analysis of the audio signal 402 involvesdetermining the actual signal level of different samples of the audiosignal 402. If the signal level of a sequence of consecutive samples ofthe audio signal 402 follows a Gaussian distribution, the sequence ofsamples is indicated as containing noise and no other signal.

The audio signal analyzing unit 500 includes the loudness adaptationunit 504, which uses the result of the loudness estimation to determineloudness adaptation indicators and introduces the loudness adaptationindicators into the audio signal 402. Loudness adaptation indicators maybe used by the gain determination unit 510 to determine an adapted gainsetting that would maintain a substantially consistent loudness level.Loudness adaptation indicators may be implemented by defining dataelements as indicating increasing or decreasing loudness. In oneexample, the loudness adaptation indicators may be time constants thatindicate whether the loudness should be increasing or decreasing. Thetime constant may also indicate whether the increase or decrease shouldbe fast or slow.

FIG. 6 illustrates the introduction of time constants into the audiosignal 402. FIG. 6 shows a sequence of audio signal samples 602, 604,and 606, which may represent a three sample sequence from a stream ofsignal samples forming the audio signal. A first time constant 608 isintroduced into the audio signal between the first sample 602 and thesecond sample 604. A second time constant 610 is introduced between thesecond sample and the third sample 606. A third time constant 612 isintroduced into the audio signal after the third sample 606. The timeconstants 608, 610, 612 indicate how the loudness should be adapted fromone sample to the next sample. The time constants 608, 610, 612 may beeither raising time constants or falling time constants. A raising timeconstant indicates how the signal gain is increased from one sample tothe next sample. A falling time constant indicates how the signal gainis decreased from one sample to the next sample.

The time constants 608, 610, 612 may be defined and implemented to adaptthe gain more rapidly for raising time constants than for falling timeconstants. The time constants may also be defined or implemented toadapt the gain in accordance with the audio signal content. If a signalpause is detected between two tracks or within a track, the audio signallevel should not be increased in order to avoid amplifying noise in thepause. When a new track starts or when the pause ends, high signallevels may follow directly after very low signal levels. The raisingtime constants of the loudness estimation may be adapted accordingly inorder to avoid too high a signal level at the beginning of a new track.The falling time constant that may be used when an audio signal leveldecreases allows for a slower decrease in signal level compared to theincrease when a rising time constant is used for an audio signalincrease. The time constants may also be implemented as adaptive timeconstants that adapt the gain more slowly as a track gets longer. Theadaptive time constants may operate similarly for the increasing anddecreasing time constants. The time constants may be implemented toprovide a smoothed loudness estimation that may be similar to the mannerin which humans perceive loudness. Peaks and dips in loudness aresmoothed out by the human auditory system. The slower varying timeconstants towards the end of an audio track help to maintain thedynamics of the audio signal. In addition, when a music signal has along runtime, a shorter reaction time for increasing loudness providesan adequate reaction to fast increases in signal levels.

The loudness adaptation indicators, implemented as the time constants inan example implementation, a gain may be determined for portions of theaudio signal and adapted as indicated by the time constants for changesin the loudness. The audio signals may be processed in blocks to saveprocessing time compared to a sample-by-sample processing. The timeconstants may be used to adjust the gain for each block. The target gainfor a block n may be attained in a linear ramp starting from the targetgain of the previous block n−1. FIG. 6 shows a graph 620 of gainincreasing and decreasing for a music signal over time. The graph 620shows a first signal block 630 of music samples at a first gain. Thefirst signal block 630 is followed by a second signal block 632 showingan increasing gain, which is followed by a third signal block 634 havinga slightly decreasing gain.

Referring back to FIG. 6, the gain determination unit 510 adapts thegain of the audio signal 402 using the estimated perceived loudness andthe loudness adaptation indicators. The loudness determination unit 502provides the estimated perceived loudness for a portion, or block, ofthe audio signal as a dB loudness equivalent (dBLEQ). The gaindetermination unit 510 also uses a predefined signal level as a targetsignal level at which the audio signal should be output. For example, adesired loudness could be set as a predefined −12 dB, which is usedbelow in FIGS. 7 and 8, although any desired loudness level may be used.The predefined level is set as a mean signal level. The gaindetermination unit 510 subtracts the determined estimated perceivedloudness from the mean signal level to calculate an adapted gain. Forexample, if the determined perceived loudness corresponds to −5 dB andthe target signal level is −12 dB full scale, the gain may be adaptedaccordingly by decreasing the gain to arrive at an average signal levelof about −12 dB.

The audio signal analyzing unit 500 includes the pause detection unit506 to detect a pause in a track or between two tracks. The pausedetection unit 506 may detect a pause by identifying a block or sequenceof blocks having audio signal samples with values that have a Gaussiandistribution, which indicates that the blocks or sequence of blocksinclude noise signals and no otherwise meaningful signals.Alternatively, or in combination, a pause may be identified bydetermining if the audio signal in a block of samples can be localizedas described above with reference to FIG. 4. The track detection unit508 determines if the pause defines a gap between two tracks. In anexample implementation, the gain is decreased for a pause if the pauseis detected over a predetermined time period. For example, the gain maybe decreased for a pause lasting between 10-100 ms, or more than 50 ms.The gain may be selectively lowered to avoid decreasing the gain duringa track in which a very short period is detected with no music signal.For example, if the audio signal includes more information and the inputsignal level is quite low, then the gain is adapted accordingly byincreasing the gain so that the audio signal covers the predeterminedrange of signal levels.

It is noted that the audio signal analyzing unit 500 in FIG. 5 includesthe loudness determination unit 502, the loudness adaptation unit 504,the pause detection unit 506, the track detection unit 508, and the gaindetermination unit 510 depicted as separate units. Those of ordinaryskill in the art would understand that the different units may beincorporated into fewer units and that the units may be combined inseveral units or even in one unit. Furthermore, the signal analyzingunit 500 may be implemented in hardware or software or by a combinationof hardware and software.

Referring back to FIG. 4, the signal output 412 of the signal analyzingunit 404 is input to the gain control unit 432. The gain control unit432 may be used to control the gain of the audio signal output to theaudio transducer as described below. The signal control unit 406includes the delay element 430 to introduce the delay into the audiosignal 402. The delay may be the time it takes the signal analyzing unit404 to determine the adapted gain.

The noise estimator 50 estimates the ambient noise level in the vehiclecabin. As shown in FIG. 1, the vehicle speed affects the noise level inthe vehicle cabin. If the vehicle is travelling at a very low velocityor is at rest, a gain adaptation as determined by the gain determinationunit may not be needed. If no gain adaptation is needed at all, the gaincontrol unit 432 may set a gain setting factor, which indicates theextent to which the output audio signal is influenced by the adaptedgain determined by the gain determination unit 504 (in FIG. 5), to 0%.The noise estimator 50 receives a vehicle velocity as input from avehicle information source. The velocity may then be used to retrieve anexpected noise level for a given velocity from a velocity-noise table440, which may be any suitable data structure for storing a relationshipbetween the vehicle velocity and the noise. The velocity-noise table 440may be a predefined table provided in data storage by the vehiclemanufacturer. The driver may or may not be given access to modify thevalues in the table 440. In an example implementation, the values in thetable 440 may be modified using a software tool configured to allow theuser to make modifications.

When the vehicle velocity is high, the ambient noise may be at 80 dB(A)as shown in FIG. 1. In this example, only 25 dB(A) remain if a thresholdof 105 dB(A) is not to be exceeded. With an ambient noise of 80 dB(A),the loudness of the audio output signal may be dynamically determined bythe gain determination unit 504 as described above with reference toFIG. 5. The gain control unit 432 may then use a factor of between 0%and 100% based on the ambient noise level to set a percentage levelindicating the extent to which the adapted gain is to be used on theaudio output signal.

The gain control unit 432 may apply a noise threshold to determine whenthe adapted gain may be applied. For example, if the estimated ambientnoise is below a predefined threshold, the adapted gain is not appliedto the output audio signal. The gain control unit 432 may also increasethe extent to which the adapted gain is applied to the output audiosignal as the estimated ambient noise increases. For example, thepercentage of adapted gain applied to the output signal may be increasedas the noise level increases.

It is noted that in the described examples, the noise estimator 420determined a noise level based on the noise relationship with velocity.However, noise based on other factors may be used, alternatively or incombination with the vehicle velocity. For example, the noise estimator420 may also measure a noise level using, for example, a microphone thatinputs a noise signal 442.

FIG. 7 shows graphs illustrating example audio signals before and afterautomatic loudness adaptation. FIG. 7 shows an example first channel 702and a second channel 704 of an audio signal before loudness adaptationis performed. The audio signal in the two channels 702, 704 includessignal samples that cover different level ranges. FIG. 7 also shows theaudio signal after loudness estimation and gain adaptation at 720 and722. The audio signal at 722 is shown with the average signal level setto −12 dB full scale while preserving the dynamic structure of the audiosignal.

FIG. 8 shows graphs illustrating another example of audio signals beforeand after automatic loudness adaptation. FIG. 8 shows a first and secondchannel 802, 804 of an audio signal in which the input level has amaximum level of −20 dB full scale. FIG. 8 also shows the audio signal820 after loudness estimation and gain adaptation. Again the dynamicstructure is preserved and the average signal level is again set to −12dB full scale at 822. If the audio signal channels 702, 704, 802, 804shown in FIGS. 7 and 8 were output as audio to the user, the user wouldhave to repeatedly adjust the volume to avoid signal levels that areunpleasantly high and in order to increase the signal for parts of theaudio signal where the signal level is too low for listening. FIGS. 7and 8 illustrate how the loudness and gain may be adapted to output anaudio signal that eliminates the need to repeatedly adjust the volume.The dynamic automatic correction of a loudness level in audio signalsthat may be attained by the automatic loudness and gain adaptation maybe used in a noisy environment, such as in a vehicle, but also inenvironments where listening to movie sounds or music should not exceeda certain loudness, such as for example, in home theatre at night.

FIG. 9 is a flowchart illustrating operation of an example method foradapting a gain of an audio signal. In an example method, an audiosignal is input at step 902. The audio signal may include entertainmentsignals input as 2.0, 5.1 or 7.1 audio signals or any other signals in asuitable audio format. An estimated perceived loudness of the audiosignal is determined at step 904. The estimated loudness may bedetermined using a psycho-acoustical localization model of the humanhearing. The audio signal may be processed in any suitable sizeportions, or blocks of audio samples. At step 906, loudness adaptationindicators such as time constants are determined. The loudnessadaptation indicators may be introduced into the audio signal at step908. The loudness adaptation indicators may be introduced between audiosignal samples as described above with reference to FIG. 6. At step 910,an adapted gain is determined for blocks of audio signal samples basedon the loudness adaptation indicators. The audio signal with adaptedgain may then be output to an audio transducer. The user may also beprovided with a control over the percentage of gain adaptation. At step912, an estimated ambient noise level is determined. The estimatedambient noise level may be determined from a relationship between thevehicle velocity and ambient noise. The estimated ambient noise levelmay also be determined from noise measurements using a microphone. Atstep 914, an output gain to apply to the output audio signal isdetermined according to the estimated ambient noise. For example, if theambient noise is below a threshold, the adapted gain is not used as theoutput gain. The output gain may be a percentage based on the ambientnoise. For example, the amount of adapted gain used increases as theambient noise level increases.

It will be understood, and is appreciated by persons skilled in the art,that one or more processes, sub-processes, or process steps described inconnection with FIGS. 1-9 may be performed by hardware and/or software.If the process is performed by software, the software may reside insoftware memory (not shown) in a suitable electronic processingcomponent or system such as, one or more of the functional components ormodules schematically depicted in FIGS. 1-9. The software in softwarememory may include an ordered listing of executable instructions forimplementing logical functions (that is, “logic” that may be implementedeither in digital form such as digital circuitry or source code or inanalog form such as analog circuitry or an analog source such an analogelectrical, sound or video signal), and may selectively be embodied inany computer-readable medium for use by or in connection with aninstruction execution system, apparatus, or device, such as acomputer-based system, processor-containing system, or other system thatmay selectively fetch the instructions from the instruction executionsystem, apparatus, or device and execute the instructions. In thecontext of this disclosure, a “computer-readable medium” is any meansthat may contain, store or communicate the program for use by or inconnection with the instruction execution system, apparatus, or device.The computer readable medium may selectively be, for example, but is notlimited to, an electronic, magnetic, optical, electromagnetic, infrared,or semiconductor system, apparatus or device. More specific examples,but nonetheless a non-exhaustive list, of computer-readable media wouldinclude the following: a portable computer diskette (magnetic), a RAM(electronic), a read-only memory “ROM” (electronic), an erasableprogrammable read-only memory (EPROM or Flash memory) (electronic) and aportable compact disc read-only memory “CDROM” (optical). Note that thecomputer-readable medium may even be paper or another suitable mediumupon which the program is printed, as the program can be electronicallycaptured, via for instance optical scanning of the paper or othermedium, then compiled, interpreted or otherwise processed in a suitablemanner if necessary, and then stored in a computer memory.

The foregoing description of implementations has been presented forpurposes of illustration and description. It is not exhaustive and doesnot limit the claimed inventions to the precise form disclosed.Modifications and variations are possible in light of the abovedescription or may be acquired from practicing the invention. The claimsand their equivalents define the scope of the invention.

What is claimed is:
 1. A method for adapting a gain of an audio signalcontaining at least two different tracks with different signal levelranges, the method comprising: dynamically determining a perceivedloudness of the audio signal based on a psychoacoustic model of a humanhearing; generating loudness adaptation indicators based on theperceived loudness and a predetermined loudness range; introducing theloudness adaptation indicators into the audio signal to generate anadapted audio signal; dynamically adapting the gain for outputting theaudio signal containing the at least two tracks based on the loudnessadaptation indicators in the adapted audio signal with an averageloudness within the predetermined loudness range; estimating an ambientnoise level; and applying the adapted gain to the audio signal accordingto the ambient noise level.
 2. The method of claim 1 further comprising:determining whether the audio signal can be localized by using asimulation of the spatial perception of the audio signal as perceived bya listener listening to the audio signal based on a binaurallocalization model; if the audio signal cannot be localized, identifyinga pause in the audio signal in which noise is a dominant part of theaudio signal; where the step of generating the loudness adaptationindicator includes generating loudness adaptation indicatorscorresponding to a decreased gain in the pause.
 3. The method of claim 1where the step of dynamically determining the perceived loudnessincludes basing the perceived loudness on signal statistics of the audioinput signal.
 4. The method of claim 1 where: each track of the audiosignal contains consecutive blocks of music signals; and the loudnessadaptation indicators are time constants describing a change of theloudness from one block to the next block.
 5. The method of claim 4where, in the step of generating the loudness adaptation indicator, araising time constant indicates an increasing loudness between twoconsecutive blocks and a falling time constant describes a decreasingloudness between two consecutive blocks, where the raising time constantindicates a faster increase in loudness than the decrease in loudnessindicated by the falling time constant.
 6. The method of claim 4 wherethe time constant is an adaptive time constant that indicates a fasterloudness adjustment at a beginning of a track than later during thetrack.
 7. The method of claim 6 further comprising: determining whetherthe audio signal can be localized using a binaural localization modelalone or in combination with a signal statistics model; detecting apause when the audio signal between two tracks cannot be localized;where the step of generating the loudness adaptation indicator includesresetting the time constant when the pause is detected between twotracks.
 8. The method of claim 1 where the step of applying the adaptedgain according to the estimated ambient noise level includes applyingthe adapted gain if the estimated ambient noise level is greater than apredefined threshold.
 9. The method of claim 1 where the step ofestimating the ambient noise level includes determining the ambientnoise from a vehicle velocity.
 10. The method of claim 1 where the stepof applying the adapted gain according to the estimated ambient noiselevel includes increasing the amount of adapted gain applied as theambient noise level increases.
 11. The method of claim 1 furthercomprising including a delay time into the audio signal before it isoutput to an audio transducer, the delay time corresponding to acalculation time to determine the adapted gain for the audio signal. 12.A system for adapting a gain of an audio signal containing at least twodifferent tracks with different signal level ranges, the systemcomprising: a loudness determination unit for determining a perceivedloudness of the audio input signal based on a psychoacoustic model of ahuman hearing; a loudness adaptation unit configured to determine aloudness adaptation indicator based on the perceived loudness and apredetermined loudness range, and to introduce the loudness adaptationindicator into the audio signal to generate an adapted gain signal; anoise estimator configured to determine an estimated ambient noiselevel; a gain determination unit configured to adapt the gain foroutputting the audio signal containing the at least two tracks based onthe loudness adaptation indicators in the adapted gain signal within thepredetermined loudness range; and a gain control unit configured todetermine an output gain to apply to the output audio signal bydetermining an extent to which the output gain is the adapted based onthe estimated ambient noise level.
 13. The system of claim 12 furthercomprising: a pause detection unit configured to determine a pause,either between the at least two different tracks or within a track, inwhich noise is the dominant part of the audio signal using a simulationof the spatial perception of the audio input signal as perceived by alistener listening to the audio signal to determine when the audiosignal cannot be localized, where the gain determination unit decreasesthe gain during the pause.
 14. The system of claim 12 where each trackof the audio signal includes consecutive blocks of audio signals, theloudness adaptation indicators generated by the loudness adaptation unitare time constants for the consecutive blocks of the audio signal, thetime constants describing a change of the loudness from one block to thenext block, where the gain determination unit adjusts the gain of theaudio signal based on the time constants.
 15. The system of claim 14where the loudness adaptation unit generates a raising time constant toindicate an increasing loudness between two consecutive blocks and afalling time constant to indicate a decreasing loudness between twoconsecutive blocks, where the raising time constant indicates a fasterincrease in loudness than the decrease in loudness indicated by thefalling time constant.
 16. The system of claim 14 where the loudnessadaptation unit determines the time constants to be adaptive timeconstants that vary faster at the beginning of a track from block toblock than later in the track.
 17. The system of claim 14 where theloudness adaptation unit resets the time constant when a pause isdetected between two tracks.
 18. The system of claim 13 furthercomprising a gain control unit configured to determine an output gainaccording to a gain correction factor indicating an amount of gaincorrection provided by the adapted gain compared to a raw gain setting.19. The system of claim 13 further comprising a delay element forintroducing a delay time into the audio signal before it is output to anaudio transducer, the delay time corresponding to a calculation time todetermine the adapted gain for the audio signal.
 20. The system of claim12 further comprising a velocity-noise table defining a relationshipbetween an ambient noise level and the vehicle velocity.
 21. The systemof claim 12 where the gain control unit is configured to apply theadapted gain when the estimated ambient noise level is above apredefined threshold.